1. Field of Invention
This invention relates to tethered wind turbines capable of gathering wind energy while airborne anchored to a land or offshore site.
2. Prior Art
Tethered wind turbines are disclosed in U.S. Pat No. 4,450,364 by Benoit, and in U.S. Pat. No. 4,491,739 by Watson. In both cases the turbine is rigidly mounted around the body of a blimp. Watson's wind turbine relies solely in the buoyancy of the blimp to offset the vertical component of the tether's anchoring force, which can only be accomplished by a large blimp. Benoit proposed installing winglets at the tips of the main rotor blades, and means of controlling their pitch angle for the purpose of generating additional lift for station keeping.
Another prior art device is disclosed in U.S. Pat. No. 4,659,940 by Shepard. He proposes that a number of airfoils be connected to wheel rims to rotate according to a cylindrical path. A kitecraft secured to an anchoring tether supports the cylindrical structure. Shepard did not consider using the airfoils to generate lift. Therefore a kitecraft is required to keep his apparatus afloat. The kitecraft solution would be impractical during periods of very light winds or would put the structure in danger during windstorms.
More recently another tethered wind turbine was disclosed in U.S. Pat. No. 6,781,254 by Roberts. He proposes that on board electrical machines are used to drive a rotor craft like a helicopter up to high altitudes where it tilts to an angle that permits the rotors, while driven by the wind, to support the rotor craft along with its tether and to generate a surplus of electrical power that is transmitted to the ground. The device uses a winch at ground level for winding and unwinding the tether to be brought down during long periods of light wind; therefore it requires a landing field, which makes its operation, especially in offshore sites, more challenging. Due to safety issues, a complex and very reliable control system is needed for its autonomous operation. There are also concerns about safety of air traffic since the anchoring tether will not be visible to air pilots or radars.